Monitoring circuit for telephone systems



Sept. 27, 1938.

- MONITORING CIRCUIT FOR TELEPHONE SYSTEMS A. E. BACHELET ET AL Filed Aug. 25, 1937 AMP l A.. BACHELET; lNVENTORS- E S. GIBSON By J v A7 ORNEV IEWTL Patented Sept. 27, 1938 UNITED STATES MONITORING CIRCUIT FOR TELEPHONE SYSTEMS Albert E. Bachelet, New York, N. Y., and Earl S.

Gibson, Ridgewood, N. J assignors to Bell Telephone Laboratories, N. Y., a corporation Application August. 25,

V V V 5 Claims.

This invention relates to signal transmission circuits and more particularly to arrangements for monitoring the same. More specifically it relates to monitoring circuits connected between transmission lines operating on different transmission levels.

Its purpose is to devise a monitoring circuit which can be connected between transmission lines of different levels and still give substantially the same level of. transmission to the receiver of the monitoring circuit, whether the signal comes fromv the one transmission line or the other.

Another purpose 'is to devise a monitoring and talking circuit adapted to impress speech on either or both of the transmission lines at the appropriate transmission level'for each line.

Other purposes will appear from the description,

which follows:

The invention is applicable to four-wire circuits in which speech signaling is carried on at voice frequencies'and will be chiefly described in such terms but it'is also and peculiarly well adapted for multi-channel telephone systems em ploying four-wire terminating sets. In such a system the different channels, have assigned to themdefinite carrier frequencies, ordinarilyabove the voice frequencyv range. In the four-wire-por tion of the circuit there are repeaters, modulators anddemodulators. At or near a terminating set wherev the message for aparticular channel has been brought to voicefrequency, the energy levels may be substantially difierent for transmission in the one direction or the other, and if. a monitoring set is connected across. the two pairs of wires at such a point, the difference in thelevel of transmission. introduces serious difiiculties which it is the purpose of our inventionto overcome. r

, The invention will be better understood by reference to the following specificationand the ac companyingv drawing, in which Fig. 1 is a diagram of the monitoring-talking circuit of the invention applied to a four-wire telephonecircuit, and

i Fig. 2 is a modification of the circuit of Fig. 1.

Referring more specifically to Fig. 1, there is shown a four wire circuit consisting of the onewaylines L1 and L2 which are coupled to a twoway two-wire circuit by means of the usual hybrid coil arrangement I I. Line L1 is indicated as providing transmission from west to east and in case signal-modulated carrier has been used, the signal has been. demodulated to voice frequency at or prior to reaching such a point as amplifier l2. Signal from the voice frequency line for. trans- Incorporated, New York, of New York 1937, Serial No. 160,757.

mission from east towest i's'shown as taking place in line L2 and subsequent to some-such. pointas indicated by the amplifier I4. is: modulated to its appropriate carrier frequency. Thus, while the circuit is adapted for use on carrier frequency, the portion of the circuitv shown in Fig. 1 relates to the voice frequency portion alone.

Commonly on arrival atthe hybrid coil I the. signal coming, from the carrier frequency portion of the system is at a relatively high leVel'com-. pared to the signal transmitted from the, voice frequency line out .to the carrier frequency portion. As an illustration, for example, atypical case would be that in which the level of trans.- mission on Lt would be +4 db. above reference level whereas at the corresponding point on L2 the transmission level would be at '13 db. If a monitoring set is now connected to the two. linesv at the points shown in the figure, it. is evident that with a symmetrical monitoringcircuit the speech level from line L1 will be 17 db.. above that received fromline L2. That is, the power will. be approximately fifty times as great fromthe one as from the other and. such difierence inlevel makes it extremely difiicult to carry on suitable monitoring. Also, in case the monitoringv circuit. is adapted for talking, then for suchsymmetrical connection in the two directions it isevidentthat the level of speech signal coming out tothe, two lines will not be appropriate to the level at which these lines are operating.

For suitable operation it is monitoring circuit that the level at the monitor's 1 receiver be substantially the same regardless of whether the monitor is listeningfon the lineLi at +4 db. or on line L2 at.-13 db. .In addition the monitor must be able to talk over eachline at approximately normal channel level .for the lines. In other words, to the monitor on the low levelor -13 db. side there should be little or no loss to the receiver. To the monitoron the. other side,- there should be 17 db. more loss than when momtoringv from the -13 db. side. To talk on the +4 db. side there should be little or no loss tothe line but to talk on the 13,db. side there should be17 db. more loss than when talking on the +4 db. side. That is, the monitoring .and talking connections bear inverse relationships. In accordance with our invention, then, the side that has the loss for monitoring is the side that does not have the loss for talking. I

In order to accomplish the desired results, we provided a hybrid coil of the inequality ratio type which we find lends itself to the reciprocal action described. Such a hy brid coil I5 is shown required. of. the.

bridged between the lines L1 and L2, this hybrid coil being so designed that there are more turns in the transformer winding n1 connected to L1 than there are in the winding 122 connected to line L2. The particular ratio to be used will depend upon the difference in level on the two lines and for the particular example shown, where there is a difference of 17 db. between the lines, the impedance ratio between these windings is fifty to one.

Across the bridging terminals of the hybrid coil I5 is connected the primary of a transformer II, the secondary of which is connected to a suitable amplifying tube l9, which is here shown as being of the pentode form. To the output of this amplifier, by means of the transformer 2|, is connected the receiver 22.

For talking purposes there is connected across the winding 24 of the hybrid coil a transmitter 25 with suitable battery 26 and associated series choke coil. In this circuit is also included the operators key 28, which normally leaves the microphone circuit open,"

In View of the relatively low impedance of the winding us there will be a substantial lack of impedance matching between this coil and the line L2. .To avoid this impedance mismatch a repeating coil 30 is introduced having a suitable turn ratio to bring about the desired impedance matching.

When monitoring on the line L1 most of the energy from the line is dissipated in winding m and only a small part is received in the receiver circuit, for the impedance of the winding n1 is substantially greater than that looking into the transformer l'l. Thus the speech received at the receiver 22 is not excessive. When monitoring on the line L2 most of the energy is dissipated in the receiver circuit and little energy is dissipated in the winding n2 for the impedance of the Winding n2 is small compared to the impedance seen when looking into the transformer l1.

Conversely, with this system as shown, when talking to line L1, most of the energy from the transmitter is transferred to the winding m and a small portion only to the winding n2. Thus the talking level to L1 from the transmitter is relatively high and the talking level to L2 is relatively low. Also with proper design, the receiver circuit with the transformer I1 is conjugate to the transmitter circuit and thus receives no energy therefrom.

There will ordinarily-be a certain amount of loss for through transmission on the lines L1 and L2 by virtue of the bridging hybrid coil l5. It is desirable that this bridging loss shall be kept to a negligible value and this can be attained by making the hybrid coil l5 one of high impedance. In case a coil of insufficiently high impedance is to be used, then the loss may still be kept to a desired small value by means of resistance pads 35 and. 36 such as shown in Fig. 2. These pads may. be allowed to remain in continuously or they may be short-circuited'when the monitor talks over either line L1 or L2. In Fig. 2 the pad 35with a relay controlled by the monitors talking circuit for short-circuiting the pad is shown in detail; the same arrangement would be used for the pad 36. The hybrid coil [5 is designed to provide a high loss between the lines L1 and L2 to prevent the transmission of undesirable echo currents between the lines L1 and L2 through the hybrid coil, tending to cause singing on the transmission circuit.

In order to obtain the most satisfactory operation of the circuit it is desirable that there should be impedance matching at the various significant points of the monitoring circuit. Thus, in looking out from the coil l5 towards L1 it. is desirable that the impedance'shall appear the same as that when looking into the coil. Such a condition would not ordinarily exist -when one introduces the pad 35 but it may be attained by bridging a resistance 31 across the input terminalsof the coil I5, this resistance being of such value that on looking away from the coil I5 the impedance shall appear to be the same as that of the coil itself. The same impedance matching is desirable on the side of the hybrid coil towards the line L2 and the resistance 38 is therefore bridged across the line adjacent to the pad 36 and of proper value to give the impedance matching. It is also desirable under some conditions that there shall be impedance matching when looking into the transmitter circuit whether the key 28 is on the listening or talking position. Such matching can be obtained by the use of resistance 39 which replaces the resistance of the microphone when key 28 is in listening position.

In all cases whether one uses the circuit of Fig; 1 or the circuit of Fig. 2 the amount of power transferred from the lines L1 or L2 to the receiver circuit will be small and for this reason we find it desirable to introduce the amplifier l9, as heretofore described. The transformer IT in the input of this amplifier may be a one-toone transformer or a step-up transformer and the transformer 2| should be step-down in order to obtain impedance matching with the receiver 22.

What is claimed is:

1. In combination, two signal transmission lines operated at difierent transmission levels, a monitoring circuit comprising a transmitter and a receiver, and means comprising an inequality ratio hybrid coil associating said monitoring circuit with said lines so as to enable transfer of signal energy from either line to said receiver at the same volume level and transfer of signal energy from said transmitter to either line at the proper line level.

2. In combination, a four-wire telephone circuit comprising oppositely directed one-way circuits for opposite directions of speech transmis-,

sion, one of said one-way circuits being operated on a higher level of speech energythan the other,

a monitoring circuit comprising a transmitter and a receiver, and an inequality ratio hybrid transformer associating said monitoring circuit with said one-way circuits so that in monitoring either line the same level of speech energy is applied to the receiver and in talking a higher level of speech energy is applied by said transmitter to the higher level line than to the lower level line.

3. In combination, a four-wire telephone circuit comprising two oppositely directed one-way circuits for opposite directions of transmission, one of said one-way circuits being operated at a higher level of speech energy than the other, a monitoring circuit comprising a transmitter and a receiver, and a three-winding transformer coupling said monitoring circuit to said one-way circuits, said transmitter being connected to one of said windings and said receiver being connected between the other two windings, one of said other two windings being connected to one one-way circuit and theother of said two windings being connected to the other one-way circuit, the windings of the transformer being relatively proportioned so that the receiver receives less energy from the one-way circuit having the higher level than from the other one-way circuit and so that the transmitter transmits more energy to the one-way circuit having the higher energy level than to the other.

4. A four-wire telephone circuit including a transmitter circuit, a monitoring circuit, two line circuits used for opposite directions of speech transmission, one of said line circuits being operated at a higher level of speech energy than the other, and. means for connecting said monitoring circuit to both of said line circuits with the same energy level obtaining at the receiver circuit regardless of whether the high or the low level line is being monitored, said means also connecting the transmitter circuit to both of the line circuits, with a higher level of speech energy being applied to the line having the higher transmission level and a lower level of speech energy being applied to the line having the lower transmission level. V

5. The combination of claim 1 characterized by the fact that the hybrid coil is of high impedance so that the loss for through transmission on the two signal transmission lines is negligible.

ALBERT E. BACHELET. EARL S. GIBSON. 

